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视频会议室灯光设计简介(英文版)(doc 8页)

视频会议室灯光设计简介(英文版)(doc 8页)

视频会议室灯光设计简介(英文版)(doc 8页)

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Designing Room Lighting For Videoconferencing

The new Design Guide from IESNA is the closest thing pro AV has to an

international standard for videoconferencing lighting design.

Source: PRO AV Magazine

Publication date: December 1, 2005

By Tim Cape, CTS-D

While the ultimate goal in videoconferencing is good interactivity between near- and

far-end rooms, one common lighting design problem can be summed up in a question

that's remained constant throughout the evolution of videoconferencing technology:

How can you provide good, comfortable lighting to optimize the camera and video

codec operation in an architectural environment rather than in a studio? Although it's a

fairly simple question, the answer can be quite complicated.

There must be some light in the room of the space you're designing just for the camera

to produce an image, but making that image look good while maintaining a good

projected video image can be easier said than done. On top of just getting the camera to

produce a decent image, you also have to be concerned about what the codec will do to

that image (particularly at low bit rates), putting more constraints on not just the

lighting design but also the interior design of the room itself — once again going

beyond the tenets of a basic AV system.

Up until now, AV professionals haven't had much consistent guidance on

videoconferencing room lighting design, but the new design guide, offered by the

Illuminating Engineering Society of North America (IESNA), gives them an authoritative

place to start.

The new design guide

In development since 2002, the “Videoconferencing Lighting Design Guide,” DG-17-05,

is now available for purchase through the IESNA website (..). It includes a wide

range of recommended criteria and general recommendations for approaches to

lighting for videoconferencing facilities. It's specifically oriented toward small- to

mid-sized videoconferencing rooms with a single camera axis (from displays to seated

participants). It doesn't specifically cover dual-axis rooms that include a presenter near

the displays or larger facilities, though much of the criteria can be applied to this wider

range of rooms.

The Guide is intended to some extent for designers both with and without a background

in lighting design.

The old design problem

Before answering the question presented above about how to optimize the camera and

video codec operation in an architectural environment, you must understand several

basic and sometimes conflicting requirements. And it's not just about the luminaires

that emit the light, it's about the room, too. The primary tasks in terms of lighting are:

Light the participants for good video. This involves limiting the variation in light level

enough to allow the camera to reproduce the full range of light and shadow in the

picture. At the same time, there needs to be enough variation in the picture to keep it

from looking “flat” with no shadow or depth.

Light the room to complement the participants. This means paying attention to the

lighting for the walls, floor, and ceiling areas that might be in the camera view.

Coordinate the room finishes from the standpoint of color, contrast, and pattern, and

how they relate to the people in the picture. This also involves consideration of the type

of finish, and how it may affect the image picked up by the camera and transmitted to

the remote site. For example, reflective, or specular, finishes such as chrome can

create problems for the camera by creating an excessively bright spot in the image.

Sometimes there are more particular constraints placed on the lighting system when

special cameras or higher than normal color rendering are needed for specialized

applications like retail, fashion, and some military environments. However, the basics

above apply to any videoconference room and are discussed in the Guide

The fundamentals

To create an environment that works well for videoconferencing, designers must

address some fundamental lighting parameters. For those in pro AV, this may mean

becoming familiar with at least the basic lighting terms you'll encounter in a lighting

design project. While the AV designer may be qualified to be the lighting designer in

some cases, it's more likely that he or she will be providing criteria to a lighting or

electrical consultant who will then design the lighting under a separate scope of work.

In either case, the AV designer needs at least some basic lighting knowledge.

To begin working with lighting, some basic lighting terms should be understood. Some

of the typical terms used in discussing and designing lighting systems are related to

those for projection and display technologies.

Illuminance is the light level incident on a surface or plane from a source or sources.

It's expressed in lux (lumens per square meter — the preferred unit of measure) or

footcandles (fc, lumens per square foot — the more obsolete unit). One footcandle

equals 10.76 lux. It's useful to note that exact conversions should be used when

reporting measurements, but when both are given as criteria a simple conversion factor

of 10 is often used instead of 10.76.

Luminance is the luminous intensity from a surface in a particular direction. Luminance

may be thought of in this discussion as the light reflected from a surface. It's expressed

as candelas per square meter (footlamberts in the more obsolete English units).

Luminaire refers to a complete lighting assembly including the housing, reflectors, and

lamps. Only when it's installed is this correctly referred to as a light fixture.

Light Reflectance Value (LRV) is a measurement often applied to painted and other

room finishes indicating the percentage of light reflected from a particular surface

independent of the color. LRV ranges from 0 to 100, with higher numbers indicating

lighter finishes and lower numbers indicating darker colors.

Luminance ratio and contrast ratio refer to the mathematical ratio of the lightest area to

the darkest area of a visual field of view. Lighting designers will often use the term

luminance ratio in discussing room environments and mostly non-electronic

illuminated surfaces. AV designers will use the term contrast ratio mostly in describing

video projectors and displays. Their fundamental definitions, however, are the same.

Color Rendering Index (CRI) is a measure of the effect a light source has on the

perceived color of objects relative to being illuminated by a reference light source. CRI

is applied to light sources and is measured on a scale of 1 to 100, where 100 is the most

accurate color rendering.

Color Temperature (or correlated color temperature, CCT) is a measure of the color

appearance of a light source. Measured on the Kelvin scale (K), so-called “warm” light

sources have a more yellowish appearance and have a lower CCT (2,000 to 3,000 K),

while “cool” sources tend to appear more white or bluish and measure above 4,000 K.

Though physics majors will recognize that the Kelvin scale is based on star

temperature in degrees Kelvin, lighting designers and the “IESNA Lighting Handbook,”

another essential industry lighting publication that explains concepts, techniques,

applications, procedures and systems, omit the word “degrees” when using the term.

The design approach

To create an effective lighting design, the fundamentals above are used within an

appropriate design approach. For videoconferencing, the design approach is based on

the concepts of photography and videography. Many will be familiar with the traditional

key, fill, and backlighting elements used in these fields. While the fill and key light

concepts apply well to the architectural/videoconferencing environment, the traditional

backlighting technique isn't as practical. Instead, the concept of background lighting is

used as defined below:

Key light is lighting from the side or above at an angle of approximately 45 degrees that

creates the primary light source for the participants. This light is normally the brightest

source and creates shadows on the faces to help with definition of facial features.

Fill light is the light that is intended to fill in the shadows created by the key light.

Without adequate fill light, shadows under the eyes, nose, and chin can appear as solid

black on camera due to the camera's limited dynamic range.

Background light is the light that covers the background in the camera's field of view,

most notably lighting the videoconference room's back and side walls.

Each of these three elements must be properly ddressed to create a good

videoconferencing picture. The difficulty comes in the application of these concepts to

an architectural environment such as a conference room where studio lighting isn't an

acceptable option.

Fortunately, there are many solutions. Several lighting manufacturers offer luminaires

designed specifically for videoconferencing, most typically for providing good key

lighting while providing good directionality to keep light off of the display area at the

front of the room. There are many luminaires that provide good, even coverage of a wall

without throwing light out away from the wall (often called “wall washers”). Standard

luminaires that are designed for typical office areas can be used if the key, fill, and

background lighting elements are addressed, but sometimes this involves a more

creative solution in coordination with the architecture and interior design.

The basic criteria

Starting the design approach, we need to apply some criteria to each element to create

a complete starting point for producing the lighting design package. There are a host of

criteria provided in the IESNA Design Guide, but the most important ones have to do

with the key, fill, and background light levels.

In general, the key light need not be above 500 lux, but should be a minimum 300 lux for

most cameras to operate within their optimum range, given typical videoconferencing

applications. Because the key light needs to be directed at an angle to the participants,

it's important to use larger surface luminaires such as 1- by 4-foot or 2- by 4-foot

fixtures where possible to reduce the potential for glare that can accompany point

source fixtures such as track lighting.

Though fill light can come from the ceiling, more often we depend on the table surface

to provide fill light on facial features, so the table surface at the participant's positions

needs to be a lighter, neutral color to reflect light onto faces that are on camera.

The third basic criterion is that the side and back walls that may be in view of the

camera need to be lit so that the luminance of the wall surfaces on camera is in line with

the luminance of the faces in the foreground. The wall luminance is created by the

combination of the light level striking the wall and the color and light reflectance value

of the wall finish.

While background lighting can be relatively simple to accomplish on the back and side

walls of a videoconference room, the background lighting is trickier when there's a

presenter who is to be on camera and is located adjacent to a projected display at the

front of the room. Most luminaires for this purpose are good at keeping light on the wall

and out of the middle of the room, but lack of side control is a problem because having

more light side-to-side can mean fewer fixtures to provide good, even wall coverage.

However, this is at odds with what you need on the wall behind a presenter and next to

a projection screen.

Don't forget to consider the display

Along with all of the other parameters, it's important to consider the type of display

being used in the videoconferencing environment. While a direct-view display such as a

CRT, plasma, or LCD monitor is very tolerant of light, front and rear projected displays

require more attention to luminaire selection and overall lighting design. Many lighting

designers are using indirect lighting approaches that work well for typical conference

rooms, classrooms, and open offices without projected displays. However, this can be

problematic when projected displays are to be used in a videoconferencing

environment because the relatively uncontrolled light in these designs may put too

much light on the display when light levels are set for camera use

The solution here is to limit the use of indirect lighting when projected displays are

involved, using more controlled, louvered fixtures instead. Point source downlighting

and track lights should be a last resort for primary participant lighting.

Other important issues

There's a plethora of other design information and criteria as well as some lighting

fundamentals included in the 25 pages that make up the new IESNA Design Guide.

Besides the issues noted in this article, lighting control, commissioning, cameras, and

codecs are discussed to create a comprehensive guide beyond what we have had

available in the pro AV industry to date. In addition, an IESNA ecommended Practice

will be developed that provides even more depth and background based on the

information in the design guide that should be completed in the next couple of years.

While the design guide isn't offered as a standard, it does contain comprehensive

criteria that can be used as pass/fail comparisons for videoconferencing lighting

applications. IESNA hopes that the new guide will provide lighting, AV, architectural,

and electrical designers with a common basis for videoconferencing room lighting

design.

SEEING THE DIFFERENCE

In the early days of videoconferencing, users were often just happy to have a picture

that was being transmitted hundreds or thousands of miles. It wasn't until they used the

technology for a while that users began to appreciate the difference between a “good”

and a “bad” picture, which frequently had to do with lighting.

The video captures above show a cool white fluorescent overhead lighting scheme in

the top photo compared to a warm white fluorescent directional lighting scheme in the

bottom photo.

Although facial features can be discerned in both cases, notice how much visual

information is “missing” in the overhead lighting arrangement due to shadows.

LIGHT DISTRIBUTION IN VIDEOCONFERENCE ROOMS

Below is a short checklist of lighting design considerations for a videoconference

system and space. Refer to the IESNA Design Guide for complete detailed criteria and

background information.

• Consider both key and fill light sources. The key light should be in the 400 to 500 lux

range when measured with a vertically oriented light meter.

• Lamps of consistent color temperature should be used throughout the room that are

compatible with the camera. The typical choice would be fluorescent lamps rated 3,000

K to 3,500 K.

• Be sure that back and side walls are evenly lit at about the same level as the

participants (this may need to vary depending on how light or dark the finish color is).

• Select solid, muted color wall finishes that are in the middle range of light

reflectance value.

• Select a table finish that's a light, neutral color.

• Use large, directional luminaires for key lighting where possible to reduce glare for

the participants. Spot fixtures may be required for presenters next to projected

displays.

• Consider the type of display being used. Projected images require lower light levels

at the screen than direct-view displays. Avoid indirect lighting schemes when projected

displays are being used.

• Provide zoned, dimmable lighting control with an AV system interface. Put any

decorative light fixtures on a separate zone so that they can be turned off during a

videoconference.

Tim Cape is a contributing editor for Pro AV, the principal consultant for Atlanta-based

technology consulting firm Technitect LLC, and co-author of “AV Best Practices,”

published by InfoComm International. He's an instructor for the InfoComm International

Audiovisual Design School and an active member of the consultant's councils for both

InfoComm International and NSCA. Contact him at